CPRNG(9) NetBSD Kernel Developer's Manual CPRNG(9)
NAMEcprng, cprng_strong_create, cprng_strong, cprng_strong32, cprng_strong64,
cprng_strong_getflags, cprng_strong_setflags, cprng_strong_ready,
cprng_strong_destroy, cprng_fast, cprng_fast32, cprng_fast64, -- crypto-
graphic pseudo-random number generators
SYNOPSIS#include <sys/cprng.h>cprng_strong_tcprng_strong_create(const char *const name, int ipl, int flags);
voidcprng_strong_destroy(cprng_strong_t *cprng);
size_tcprng_strong(cprng_strong_t *const cprng, void *buf, size_t len,
int blocking);
size_tcprng_fast(void *buf, size_t len);
uint32_tcprng_strong32(void);
uint64_tcprng_strong64(void);
uint32_tcprng_fast32(void);
uint32_tcprng_fast64(void);
intcprng_strong_getflags(cprng_strong_t *const cprng);
voidcprng_strong_setflags(cprng_strong_t *const cprng, int flags);
#define CPRNG_MAX_LEN 524288
typedef struct _cprng_strong {
kmutex_t mtx;
kcondvar_t cv;
struct selinfo selq;
NIST_CTR_DRBG drbg;
int flags;
char name[16];
int reseed_pending;
rndsink_t reseed;
} cprng_strong_t;
DESCRIPTION
The cprng family of functions supply randomness to callers within the
NetBSD kernel. They replace the arc4random(9) and rnd_extract_data(9)
functions for this purpose. The cprng functions provide stream genera-
tors automatically keyed (and if necessary rekeyed) from the kernel
entropy pool. The NetBSD kernel no longer supports direct reading from
the kernel entropy pool; all access is mediated by the cprng functions.
The ``strong'' family of functions supply cryptographically strong random
numbers suitable for keying crypto systems and similar purposes. Calls
to rnd_extract_data(9) should be replaced with calls to cprng_strong.
The ``fast'' family of functions supply less strong random numbers, suit-
able for initialization vectors, nonces in certain protocols, and other
similar purposes, using a faster but less secure stream-cipher generator.
stream-cipher generator. Calls to arc4random(9) should be replaced with
calls to cprng_fast32, and calls to arc4randbytes(9) should be replaced
with calls to cprng_fast.
A single instance of the cprng_fast generator serves the entire kernel.
A single, well-known instance of the cprng_strong generator, kern_cprng,
may be used by any in-kernel caller, but new separately-keyed instances
of the cprng_strong generator can also be created by calling
cprng_strong_create.
FUNCTIONScprng_strong_create(name, ipl, flags)
Create an instance of the cprng_strong generator. This generator
implements the NIST SP 800-90 CTR_DRBG with AES128 as the block
transform. The name argument is used to "personalize" the CTR_DRBG
according to the standard, so that its initial state will depend
both on keying material from the entropy pool and also on the per-
sonalization string (name). The ipl argument specifies the inter-
rupt priority level for the mutex which will serialize access to
the new instance of the generator (see spl(9)). The flags argument
controls the behavior of the generator:
CPRNG_INIT_ANY Perform initial keying of the generator from the
entropy pool even if the current estimate of
entropy in the pool is less than the required num-
ber of key bits for the generator.
CPRNG_REKEY_ANY When rekeying of the generator is required, key
the generator from the entropy pool even if the
current estimate of entropy in the pool is less
than the required number of key bits for the gen-
erator.
CPRNG_USE_CV Perform a cv_broadcast(9) operation on the "cv"
member of the returned cprng_strong_t each time
the generator is successfully rekeyed. If thisflag is set, the generator will sleep whenrekeying is needed, and will therefore alwaysreturn the requested number of bytes.
Creation will succeed even if key material for the generator is not
available. In this case, the first request to read from the gener-
ator may cause rekeying.
cprng_strong_destroy(cprng)
Destroy an instance of the cprng_strong generator.
cprng_strong(cprng, buf, len, blocking)
Fill memory location buf with len bytes from the generator cprng.
The blocking argument controls the blocking/non-blocking behavior
of the generator: if it is set to FNONBLOCK, the generator may
return less than len bytes if it requires rekeying. If the
CPRNG_USE_CV flag is set on the generator, the caller can wait on
cprng->cv for notification that the generator can again supply
bytes. A maximum of CPRNG_MAX_LEN bytes may be requested at once;
this is a restriction of the CTR_DRBG specification.
cprng_strong32(cprng)
Generate 32 bits using cprng_strong generator cprng.
cprng_strong64(cprng)
Generate 64 bits using cprng_strong generator cprng.
cprng_strong_getflags(cprng)
Get the flags currently in use by generator cprng.
cprng_strong_setflags(cprng, flags)
Set the flags on generator cprng to flags.
cprng_fast(buf, len)
Fill memory location buf with len bytes from the fast generator.
cprng_fast32()
Generate 32 bits using the fast generator.
cprng_fast64()
Generate 64 bits using the fast generator.
CODE REFERENCES
The cprng API is implemented by sys/kern/subr_cprng.c and
sys/sys/cprng.h. The ``strong'' generator uses the CTR_DRBG implementa-
tion in sys/crypto/nist_ctr_drbg. The ``fast'' generator uses the
arc4random implementation in sys/lib/libkern/arc4random.c.
SEE ALSOcondvar(9), rnd(9), spl(9)
Elaine Barker and John Kelsey, Recommendation for Random NumberGeneration Using Deterministic Random Bit Generators (Revised), NationalInstitute of Standards and Technology, 2011, NIST Special Publication
800-90A, Rev 1.
HISTORY
The cprng family of functions first appeared in NetBSD 6.0.
NetBSD 6.0 December 17, 2011 NetBSD 6.0

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